Refrigerating system and method for controlling the same

ABSTRACT

A refrigerating system according to the invention comprises a refrigerant circuit ( 2 ) having at least one compressor ( 4 ), a condenser/gascooler ( 6 ), an intermediate pressure container ( 8 ), at least one evaporator ( 10 ) and a respective expansion device ( 12 ) before said at least one evaporator ( 10 ), and refrigerant pipes ( 14 ) connecting said elements and circulating a refrigerant therethrough; a high pressure regulating device ( 16 ) between the condenser/gas-cooler ( 6 ) and the intermediate pressure container ( 8 ), expanding the refrigerant from a high pressure level to an intermediate pressure level; an intermediate pressure sensor sensing the intermediate pressure level; and a control unit ( 18 ) controlling the high pressure regulating device ( 16 ). The control unit ( 18 ) in operation limits the maximum refrigerant flow through the high pressure regulating device ( 16 ) to a maximum flow value F Max , if the sensed intermediate pressure level exceeds a predetermined threshold value P IntTh .

This application is entitled to the benefit of, and incorporates byreference essential subject matter disclosed in PCT Application No.PCT/EP2007/008818 filed on Oct. 10, 2007.

BACKGROUND OF THE INVENTION

1. Technical Field

The invention relates to a refrigerating system and to a method forcontrolling a refrigerating system.

2. Background Information

Refrigerating systems using CO₂ as a refrigerant are well known in theart. It is also known that, in these systems, the refrigerant comingfrom the condenser/gascooler is expanded to an intermediate pressurelevel via a throttle valve, before being expanded further by a furtherthrottle valve before an evaporator. The refrigerant pressure in thecondenser/gascooler is commonly regulated by setting the degree ofaperture of said valve between the condenser/gascooler and theintermediate pressure part of the system. For example, in the case of apressure in the condenser/gascooler exceeding a reference pressurevalue, the pressure is released via said valve. This can lead to anincrease of the intermediate pressure level over a critical value, suchthat the refrigerant has to be released into the environment via safetyvalves. This loss of refrigerant will lead to a performance reduction ofthe refrigerating system and can cause damage to the system.

Accordingly, it would be beneficial to provide a refrigerating systemhaving an increased performance and durability.

SUMMARY OF THE INVENTION

Exemplary embodiments of the invention include a refrigerating systemincluding a refrigerant circuit having at least one compressor, acondenser/gascooler, an intermediate pressure container, at least oneevaporator and a respective expansion device before said at least oneevaporator, and refrigerant pipes connecting said elements andcirculating a refrigerant therethrough; a high pressure regulatingdevice between the condenser/gascooler and the intermediate pressurecontainer, expanding the refrigerant from a high pressure level to anintermediate pressure level; an intermediate pressure sensor sensing theintermediate pressure level; and a control unit controlling the highpressure regulating device. The control unit limits in operation themaximum refrigerant flow through the high pressure regulating device toa maximum flow value F_(Max) if the sensed intermediate pressure levelexceeds a predetermined threshold value P_(IntTh).

Exemplary embodiments of the invention further include a method forcontrolling a refrigerating system comprising a refrigerant circuithaving at least one compressor, a condenser/gascooler, an intermediatepressure container, at least one evaporator and a respective expansiondevice before said at least one evaporator, and refrigerant pipesconnecting said elements and circulating a refrigerant therethrough; ahigh pressure regulating device between the condenser/gascooler and theintermediate pressure container, expanding the refrigerant from a highpressure level to an intermediate pressure level; and an intermediatepressure sensor sensing the intermediate pressure level; the methodcomprising the step of controlling the high pressure regulating deviceby limiting the maximum refrigerant flow through the high pressureregulating device to a maximum flow value F_(Max) if the sensedintermediate pressure level exceeds a predetermined threshold valueP_(IntTh).

Embodiments of the invention are described in greater detail below withreference to the figures, wherein:

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of an exemplary refrigerant circuit inaccordance with the present invention;

FIG. 2 shows an exemplary function of the maximum allowable flow throughthe high pressure regulating device depending on the intermediatepressure level; and

FIG. 3 shows an exemplary course of the intermediate pressure level andthe maximum allowable flow through the high pressure regulating deviceover time.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a refrigerant circuit 2 in accordance with an embodiment ofthe present invention. The refrigerant circuit 2 includes the fourstandard components of refrigerating circuits: a compressor 4, acondenser/gascooler 6, an expansion device 12, and an evaporator 10. Inthe embodiment shown the refrigerant circuit 2 comprises a set of threecompressors 4 and three evaporators 10, with expansion devices 12associated respectively. The elements mentioned are connected byrefrigerant pipes 14. The refrigerant circuit 2 of FIG. 1 also comprisesan intermediate pressure container 8 and a high pressure regulatingdevice 16, which is located between the condenser/gascooler 6 and theintermediate pressure container 8.

This high pressure regulating device 16 allows for a controlled releaseof pressure from the condenser/gascooler 6 to the intermediate pressurecontainer 8. For this purpose the flow through the high pressureregulating device 16 is set. The flow can be set by adjusting the degreeof aperture of the high pressure regulating device 16. The high pressureregulating device 16 may be a regulating valve. As a matter of course,any other device known to the skilled person to be appropriate for theabove described regulating operation may be used as well.

If the refrigerant circuit 2 is operated at relatively high pressure inthe refrigerant pipe between the set of compressors 4 and thecondenser/gascooler 6, e.g. at a pressure of more than 73,83° C., therefrigerant reaches a transcritical region. In this transcriticaloperation, the condenser 6 works as a gascooler. In the following theterm “condenser” shall be understood as to include both the meaning ofcondenser and gascooler.

A control unit 18 controls said high pressure regulating device 16, andit is connected to a high pressure sensor (not shown) arranged at thecondenser 6 and to an intermediate pressure sensor (not shown) at theintermediate pressure container 8 sensing the intermediate pressurelevel in order to provide the control unit 18 with a momentaryintermediate pressure value P_(Int).

The intermediate pressure sensor may be located somewhere in theintermediate pressure portion of the refrigerant circuit 2, i.e. in thepipe between the high pressure regulating device 16 and the intermediatepressure container 8, in the intermediate pressure container 8, or inthe pipe between the intermediate pressure container 8 and the at leastone expansion device 12 before the at least one evaporator 10. In theembodiment of FIG. 1, the intermediate pressure sensor is located in theintermediate pressure container 8, preferable in a vicinity of theoutlet, from where the refrigerant pipe 14 carries the refrigeranttowards the evaporator 10.

The high pressure sensor, which senses the high pressure level, may belocated somewhere in the high pressure portion of the refrigerantcircuit 2. In the embodiment of FIG. 1 the high pressure sensor islocated in the condenser 6. Said high pressure sensor can either measurethe pressure directly or deduce the pressure from the exit temperatureof the condenser 6.

FIG. 1 additionally shows a gaseous refrigerant refeed pipe 20connecting the gas space of the intermediate pressure container 8 to thesuction line of the set of compressors 4. Associated therewith,preferably arranged within the refeed pipe 20, is an intermediatepressure regulating device 22. As the intermediate pressure container 8in operation separates liquid refrigerant from gaseous refrigerant, thegaseous refrigerant refeed pipe 20 is adapted to carry said gaseousrefrigerant away from said intermediate pressure container 8. Havingthis refeed pipe 20 in place and controlling the intermediate pressureregulating device 22 accordingly allows for keeping the intermediatepressure level constant, which increases efficiency of the refrigeratingsystem. Expansion devices 12 and evaporators 10 operate better when fedby constant pressure from intermediate pressure container 8. The gaseousrefrigerant is fed back from the intermediate pressure container 8 tothe input side of the at least one compressor 4.

FIG. 2 shows an exemplary function of the maximum allowable flow F_(max)through the high pressure regulating device 16 depending on theintermediate pressure level P_(Int).

The axes of the graph of FIG. 2 represent the intermediate pressurelevel P_(Int) and the maximum allowable flow F_(Max).

The course of the maximum allowable flow F_(Max) is constant for anintermediate pressure level P_(Int) up to a threshold value P_(IntTh)and decreases in a linear manner with a further increase of theintermediate pressure level P_(Int).

The shown dependency can be expressed with the formula:

F _(Max) =F _(TecMax)(1−C _(P)(P _(Int) −P _(IntTh))),

wherein F_(TecMax) is the maximum flow the high pressure regulatingdevice 16 technically supports and C_(P) is a proportionality constant.

FIG. 3 shows two synchronized graphs, the upper graph depicting anexemplary course of the maximum allowable flow F_(Max) through the highpressure regulating device 16 over time as calculated by the controlunit 18, and the lower graph depicting a corresponding course of theintermediate pressure level P_(Int) over time.

The intermediate pressure level P_(Int) increases starting below areference intermediate pressure value P_(IntRef), it exceeds thethreshold value P_(IntTh), and it reaches its peak lying below themaximum allowable intermediate pressure value P_(IntMax). Then theintermediate pressure level P_(Int) decreases, finally reaching a valuearound the reference intermediate pressure value P_(IntRef).

The maximum allowable flow F_(Max) through the high pressure regulatingdevice 16 is constant at the maximum technically possible flow valueF_(TecMax), when the intermediate pressure level P_(Int) is below thethreshold value P_(IntTh), and decreases in proportion to the amount theintermediate pressure level P_(Int) exceeds the threshold valueP_(IntTh), and it increases again with falling intermediate pressurelevel P_(Int).

The control of the refrigerating system according to exemplaryembodiments of the invention is explained as follows.

The intermediate pressure level P_(Int) is used as an input to thecontrol unit 18. Based on the intermediate pressure level P_(Int), thecontrol unit 18 calculates a maximum tolerable flow F_(Max) through or amaximum allowable degree of aperture of the high pressure regulatingdevice 16. This makes the release of pressure from the high pressurelevel, i.e. the pressure level in the condenser 6 and before the highpressure regulating device 16, dependent on the intermediate pressurelevel P_(Int). The control unit 18 sets a maximum tolerable flow valueF_(Max) once the intermediate pressure level exceeds P_(Int) a presetthreshold value P_(IntTh); i.e. the control unit 18 allows a refrigerantflow through expansion device 16 as high as technically possible underthe momentary system conditions, as long as the intermediate pressurelevel P_(Int) stays below said predetermined threshold value P_(Inth).This behavior takes into account that a release of pressure from thecondenser 6 into the intermediate pressure container 8 does not affectthe safety of the system in the intermediate pressure region, as long asthe threshold value P_(IntTh) for the intermediate pressure levelP_(Int) is not reached.

In the exemplary embodiment, the threshold value P_(IntTh) is set to bebetween a reference intermediate pressure value P_(IntRef) and acritical intermediate pressure level, herein also referred to as maximumallowable intermediate pressure value P_(IntMax). This allows for norestriction of the flow through the high pressure regulating device 16due to the intermediate pressure level P_(Int), as long as saidintermediate pressure level P_(Int) is around the desired referencevalue P_(IntRef). In case the intermediate pressure level P_(Int) leavesthe region around the reference intermediate pressure value P_(IntRef)and moves towards a critical intermediate pressure level, the limitingoperation by the control unit 18 kicks in. A threshold value P_(IntTh)lying between a fourth and a half, particularly around a third, of therange from the reference intermediate pressure value P_(IntRef) to themaximum allowable intermediate pressure value P_(IntMax) is used in theexemplary embodiment.

As discussed above, the maximum refrigerant flow through the highpressure regulating device 16 is limited by the control unit 18, in casea threshold value P_(InTh) is exceeded by the intermediate pressurelevel P_(Int). In the exemplary embodiment, the more the intermediatepressure level P_(Int) exceeds the threshold value P_(IntTh), the morerestrictive is the limit imposed on the refrigerant flow through thehigh pressure regulating device 16 by the control unit 18. Incombination with refrigerant constantly leaving the intermediatepressure portion of the refrigerant circuit 2 through the expansiondevices 12 and the intermediate pressure regulating device 22, a slightreduction of the maximum tolerable flow F_(Max) may suffice to stop afurther increase in or reduce the intermediate pressure level P_(Int)when only slightly exceeding the threshold value P_(IntTh). For afurther increased intermediate pressure level P_(Int), the reduction ofthe maximum tolerable flow F_(Max) is increased as well. Up until thethreshold value P_(IntTh) the maximum tolerable flow F_(Max) is at a100% of the flow that is technically possible under the momentary systemconditions. With the intermediate pressure level P_(Int) exceeding thethreshold value P_(IntTh), the maximum tolerable flow F_(Max) isdecreased in a linear manner with a further increase of the intermediatepressure level P_(Int), as illustrated in FIG. 2.

The proportionality constant C_(P) determines the slope of thedecreasing portion of the function depicted in FIG. 2. C_(P) can bechosen according to the desired control behavior. If, for example, acomplete closure of the high pressure regulating device 16 is wantedwhen the critical intermediate pressure level is reached, C_(P) isdimensioned, such that the product of C_(P) and (P_(IntMax)−P_(IntTh))equals to one. C_(P) can also be chosen larger, in order to preventreaching of the critical intermediate pressure level, since the highpressure regulating device 16 is shut down completely before thecritical intermediate pressure level is reached. C_(P) can also bechosen smaller, in order to make the control of the refrigerant flowthrough the high pressure regulating device 16 less restrictive, whichwill allow for more efficient release of pressure from the condenser 6.

The proportionality constant C_(P) does not necessarily have a fixedvalue. It can be adjusted when installing the refrigerating system,taking into account operating conditions, such as location, operationenvironment, etc. An adjustment of C_(P) can also be used to compensatefor changes in the environment of the control unit 18 during thelifespan of the refrigerating system, so that the whole system does notnecessarily have to be recalibrated.

As can be seen from the two graphs of FIG. 3, the maximum tolerable flowF_(Max) is the maximum technically possible flow value F_(TecMax), aslong as the intermediate pressure level P_(Int) stays below thethreshold value P_(IntTh), i.e. in a region around the referenceintermediate pressure value P_(IntRef). As soon as the intermediatepressure level P_(Int) exceeds the predetermined threshold valueP_(IntTh), the maximum allowable flow F_(Max), which is set by thecontrol unit 18, is decreased. The mapping of the intermediate pressurelevel P_(Int) onto the maximum allowable flow F_(Max), which is thebasis for creating the maximum flow curve as shown in the upper graph ofFIG. 3, is done using a function of the sort of the function depicted inFIG. 2.

Instead of the function depicted in FIG. 2 a plurality of otherfunctions may be used to map the intermediate pressure level P_(Int)onto the maximum flow value F_(Max) through the high pressure regulatingdevice 16. For example, a step function can be thought of, in which aplurality of additional threshold values exists, which are greater thanP_(IntTh). For each of the threshold values, the maximum tolerable flowF_(Max) is decreased by a discrete value. This will result in amonotonically decreasing, step-like function. Also, one may think of alogarithmic decrease of the maximum tolerable flow F_(Max) with theintermediate pressure level P_(Int) increasing beyond the thresholdvalue P_(IntTh), which allows for a slight reduction in the maximumtolerable flow F_(Max) for values of the intermediate pressure levelP_(Int) exceeding the threshold value P_(IntTh) by only a little bit anda more and more drastic reduction the more the intermediate pressurelevel P_(Int) exceeds the threshold value P_(IntTh). From the previousexamples it is apparent to a person skilled in the art that there aremany options for functions that may be used by the control unit 18 tocalculate the maximum flow value F_(Max).

In the embodiment shown in FIG. 1, control unit 18 not only sets themaximum flow value F_(Max) and controls the high pressure regulatingdevice 16 accordingly, but also calculates the desired actual flow valuethrough high pressure regulating device 16 and controls said deviceaccordingly. This could also be done by separate controls. For exampleone control could limit the degree of aperture of a regulating valveemploying a stopping mechanism, whereas a second control is responsiblefor actually changing the degree of aperture. However, in the presentembodiment the functionality is combined in a single control unit 18.

The actual flow through high pressure regulating device 16, which cannot exceed the maximum flow value F_(Max) calculated by control unit 18,is set by control unit 18 dependent on the high pressure level. The highpressure level is obtained by the high pressure sensor. By controllingthe high pressure regulating device 16 depending on the high pressurelevel, the high pressure level is regulated in order to achieve adesired cooling of the refrigerant in the condenser 6.

In order to avoid pressure levels that are possibly damaging to therefrigerating system, the high pressure portion as well as theintermediate pressure portion of the refrigerant circuit 2 may compriserespective safety valves (not shown). For the high pressure portion ofthe refrigerant circuit 2, the safety valve may release pressure in casethe pressure release through the high pressure regulating device 16 isnot sufficient. For the intermediate pressure portion, an insufficientpressure release through the intermediate pressure regulating device 22or the expansion devices 12 can be compensated by the safety valvestructure. High pressure portion as well as intermediate pressureportion may comprise one or a plurality of safety valves. Possiblydamaging or in any other way critical pressure values may thereby beprevented.

Other measures may alternatively or in addition be taken to prevent thehigh pressure level from reaching a critical level. When approaching acritical pressure value in the high pressure portion, the flow limitingcontrol through the high pressure regulating device 16, carried out bycontrol unit 18, may be suspended. In other words, a critical scenarioin the high pressure portion of the refrigerating circuit 2 may overridethe flow limit set depending on the intermediate pressure level P_(Int).

Also, for the high pressure level approaching a critical value, theperformance of the compressor 4 may be degraded or one of a plurality ofcompressors 4 may be switched off. This allows for an overall pressurerelease in the refrigerating circuit 2.

In an embodiment of the invention, CO₂ is used as a refrigerant. Whenusing CO₂, typical pressure values are 50-100 bar for the high pressurelevel, 30-40 bar for the intermediate pressure level P_(Int), and 20-35bar for the portion between the evaporator and the input to thecompressor. The high pressure safety valve may open between 115-120 bar,whereas the intermediate pressure safety valve may open at approximately40 bar. As a matter of course any other refrigerant known to the skilledperson may be used as well.

Exemplary embodiments of the invention, as described above, allow forlimiting the flow through the high pressure regulating device dependenton the intermediate pressure level P_(Int), thus avoiding theintermediate pressure level P_(Int) to reach a critical intermediatepressure level. Therefore they allow for preventing an otherwiseoccasionally necessary release of refrigerant from the intermediatepressure portion of the refrigerant circuit into the environment viasafety valves. The exemplary embodiments furthermore allow for providinga structure for a control that bases its regulating operation on boththe high and the intermediate pressure level. This coupling of highpressure regulation and intermediate pressure regulation leads to agreater overall system efficiency, safety, and durability.

As described above, the predetermined threshold value P_(IntTh) may bebetween a reference intermediate pressure value P_(IntRef) and a maximumallowable intermediate pressure value P_(IntMax). This allows for thepossibility of a not limited pressure release from the high pressurelevel to the intermediate pressure level P_(Int), as long as theintermediate pressure level P_(Int) is around a predetermined referenceintermediate pressure value P_(IntRef). It also allows for the flowlimiting operation to set in before a critical intermediate pressurevalue is reached. More particularly, the predetermined threshold valueP_(IntTh) can be set according to the formulaP_(IntTh)=P_(IntRef)+C_(Th) (P_(IntMax)−P_(IntRef)), wherein C_(Th) is aconstant lying between 0.25 and 0.5.

It is possible for the control unit in operation to set a maximum flowvalue F_(Max) using a monotonically decreasing function of theintermediate pressure level P_(Int). The flow through the high pressureregulating device, which increases the intermediate pressure levelP_(Int), is thereby more and more limited, the more the intermediatepressure level P_(Int) increases from the threshold value P_(IntTh)towards the maximum allowable intermediate pressure value P_(IntMax).The flow can be decreased gradually in a linear fashion, when themaximum flow value F_(Max) is set according to the formulaF_(Max)=F_(TecMax) (1−C_(P) (P_(Int)−P_(IntTh))), wherein F_(TecMax) isthe maximum flow the high pressure regulating device technicallysupports and C_(P) is a proportionality constant. The proportionalityconstant C_(P) can be adaptable to adjust the refrigerating system tovarious operating conditions. This allows for adjusting therefrigerating system to its location and environment, particularly uponinstallation. It also provides means for additional adjustmentsthroughout the lifespan of the system.

The refrigerant circuit can additionally comprise a high pressure sensorsensing the high pressure level. This allows for additional controloptions for the high pressure regulating device. These additionalcontrol measures may be implemented in the control unit. It is possiblethat the control unit in operation sets the refrigerant flow through thehigh pressure regulating device to an actual flow value based on thesensed high pressure level under the condition that the maximum flowvalue F_(Max) is not exceeded. Thereby an unwanted release of pressurefrom the high pressure level can be prevented in order to build up thehigh pressure level to a reference value.

It is also possible that the refrigerant flow through the high pressureregulating device is adjusted by changing the degree of aperturethereof. Control of the degree of aperture of the regulating device, forexample a regulating valve, directly controls the flow.

As explained above, the intermediate pressure container may in operationseparate liquid refrigerant from gaseous refrigerant. The refrigerantcircuit may comprise a gaseous refrigerant refeed pipe connecting a gasspace of the intermediate pressure container with the at least onecompressor and an intermediate pressure regulating device arrangedwithin the gaseous refrigerant refeed pipe. This allows for the gaseousrefrigerant being carried away from the intermediate pressure containertowards the inlet of the at least one compressor, bypassing the at leastone expansion device and the at least one evaporator, through whichliquid refrigerant is flown.

It is furthermore possible that the condenser and the intermediatepressure container each have at least one safety valve associatedtherewith, the safety valves being adapted to release refrigerant whenthe pressure reaches a critical level in the respective parts of therefrigerating system. This allows for having an emergency system inplace, in case critical values of the high pressure level and/or theintermediate pressure level P_(Int) can not be sufficiently controlledby the high pressure regulating device and the intermediate pressureregulating device, respectively.

Moreover, it is possible in case of the high pressure level approachinga critical value that the control unit stops limiting the maximumrefrigerant flow F_(MS) through the high pressure regulating device.Additionally/alternatively in case of the high pressure levelapproaching a critical value, the control unit may subsequently switchoff compressor stages of the at least one compressor. These measuresallow for alleviating the high pressure level without making use of thedrastic measure of opening the safety valves.

As mentioned before, the refrigerant may be CO₂.

With the method for controlling a refrigerating system according toexemplary embodiments of the invention, as described above, the sameadvantages can be attained as with the refrigerating system. This methodcan be developed further by method steps corresponding to the featuresas described with regard to the refrigerating system. In order to avoidredundancy such embodiments and developments of the method forcontrolling a refrigerating system are not repeated.

While the invention has been described with reference to exemplaryembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed, but that theinvention will include all embodiments falling within the scope of theappended claims.

1. Refrigerating system having a refrigerant circuit comprising: atleast one compressor, a condenser/gascooler, an intermediate pressurecontainer, at least one evaporator and a respective expansion devicebefore said at least one evaporator, and refrigerant pipes connectingsaid elements and circulating a refrigerant therethrough; a highpressure regulating device between the condenser/gascooler and theintermediate pressure container, expanding the refrigerant from a highpressure level to an intermediate pressure level; an intermediatepressure sensor sensing the intermediate pressure level; and a controlunit controlling the high pressure regulating device, wherein thecontrol unit in operation limits the maximum refrigerant flow throughthe high pressure regulating device to a maximum flow value F_(Max) ifthe sensed intermediate pressure level exceeds a predetermined thresholdvalue P_(IntTh).
 2. Refrigerating system according to claim 1, whereinthe predetermined threshold value P_(IntTh) is between a referenceintermediate pressure value P_(IntRef) and a maximum allowableintermediate pressure value P_(IntMax).
 3. Refrigerating systemaccording to claim 2, wherein the predetermined threshold valueP_(IntTh) is set according to the formula:P _(IntTh) =P _(IntRef) +C _(Th)(P _(IntMax) −P _(IntRef)), whereinC_(Th) is a constant lying between 0.25 and 0.5.
 4. Refrigerating systemaccording to claim 1, wherein the control unit in operation sets themaximum flow value F_(Max) using a monotonically decreasing function ofthe intermediate pressure level P_(Int).
 5. Refrigerating systemaccording to claim 4, wherein the maximum flow value F_(Max) is setaccording to the formula:F _(Max) =F _(TecMax)(1−C _(P)(P ₁ nt−P _(IntTh))), wherein F_(TecMax)is the maximum flow the high pressure regulating device technicallysupports and C_(P) is a proportionality constant.
 6. Refrigeratingsystem according to claim 5, wherein the proportionality constant C_(P)is adaptable to adjust the refrigerating system to various operatingconditions.
 7. Refrigerating system according to claim 1, wherein therefrigerant circuit additionally comprises a high pressure sensorsensing the high pressure level.
 8. Refrigerating system according toclaim 7, wherein the control unit in operation limits the refrigerantflow through the high pressure regulating device to an actual flow valuebased on the sensed high pressure level under the condition that themaximum flow value F_(Max) is not exceeded.
 9. Refrigerating systemaccording to claim 1, wherein the refrigerant flow through the highpressure regulating device is adjusted by changing the degree ofaperture thereof.
 10. Refrigerating system according to claim 1, whereinthe intermediate pressure container in operation separates liquidrefrigerant from gaseous refrigerant.
 11. Refrigerating system accordingto claim 10, wherein the refrigerant circuit also comprises a gaseousrefrigerant refeed pipe connecting a gas space of the intermediatepressure container with the at least one compressor and an intermediatepressure regulating device arranged within the gaseous refrigerantrefeed pipe.
 12. Refrigerating system according to claim 1, wherein thecondenser/gascooler and the intermediate pressure container each have atleast one safety valve associated therewith, the safety valves beingadapted to release refrigerant when the pressure reaches a criticallevel in the respective parts of the refrigerating system. 13.Refrigerating system according to claim 1, wherein, in case of the highpressure level approaching a critical value, the control unit stopslimiting the maximum refrigerant flow F_(Max) through the high pressureregulating valve.
 14. Refrigerating system according to claim 1,wherein, in case of the high pressure level approaching a criticalvalue, the control unit subsequently switches off compressor stages ofthe at least one compressor.
 15. Refrigerating system according to claim1, wherein the refrigerant is CO₂.
 16. Method for controlling arefrigerating system comprising a refrigerant circuit having at leastone compressor, a condenser/gascooler, an intermediate pressurecontainer, at least one evaporator and a respective expansion devicebefore said at least one evaporator, and refrigerant pipes connectingsaid elements and circulating a refrigerant therethrough; a highpressure regulating device between the condenser/gascooler and theintermediate pressure container, expanding the refrigerant from a highpressure level to an intermediate pressure level; and an intermediatepressure sensor sensing the intermediate pressure level; the methodcomprising the step of controlling the high pressure regulating deviceby limiting the maximum refrigerant flow through the high pressureregulating device to a maximum flow value F_(Max) if the sensedintermediate pressure level exceeds a predetermined threshold valueP_(IntTh).